Several recent investigations of dual-energy computed tomography (References 1 to 5) involve changing the X-ray tube voltages for respective scans, for example, from 100 kVp to 140 kVp (kVp=kilovolts peak). Because of differences in chemical composition and effective atomic number of the scanned tissues, the energy dependence of the computed tomography number will be different for various tissues. Thus, high Z elements like calcium and iodine may be identified, and types of soft tissues and tumors may be differentiated.
The previously developed dual-kVp system requires two separate scans for each slice, causing a problem of registration, since the patient may move between the scans. Since two scans are required, the procedure is relatively time-consuming. Also, energy discrimination is somewhat unsatisfactory.
As a result of a preliminary search, the following prior U.S. patents appear to be of interest:
Hounsfield, U.S. Pat. No. 4,052,619 PA0 Barbier, U.S. Pat. No. 4,053,779 PA0 Allemand, U.S. Pat. No. 4,055,767 PA0 Pasedach, U.S. Pat. No. 4,064,393 PA0 Ruhrnschopf et at, U.S. Pat. No. 4,065,397 PA0 Frogatt, U.S. Pat. No. 3,996,467.
The background of the present invention is further disclosed by the following reference publication:
1. R. A. Rutherford, B. R. Pullan, I. Isherwood, "Measurement of Effective Atomic Number and Electron Density Using an EMI Scanner", 11:15-21, Neuroradiol., Jan. 1976.
2. M. J. Derggren, G. T. Herman, P. Ruefsegger, et al, "Computer Assisted Tomography of Small Quantities of Selected Materials" (Abstract), J. Comput.Assisted Tomog., 1:254-255, Apr. 1977.
3. L. M. Zatz, "The Effect of the kVp Level on EMI Values, Selective Imaging of Various Materials with Different kVp Settings", Radiology, 119:683-688, June 1976.
4. R. E. Latchaw, J. T. Payne, L. H. A. Gold, "Effective Atomic Number and Electron Density as Meaured with a CT Scanner: Computation and Correlation with Brain Tumor Histology". Presented at the 15th Annual Meeting of the American Society of Neuroradiology, Bermuda, Mar. 27-31, 1977.
5. L. Dubal, U. Wiggli, "Tomochemistry of the Brain", J. Comput. Assisted Tomog., 3:300-307, July 1977.
6. R. E. Alvarez, A. Macoviski, "Energy-selective Reconstructions in X-ray Computerized Tomograph.", Phys. Med. Biol. 21:733-744, Sept. 1976.
7. E. C. McCullough, H. L. Baker, O. W. Houser, et al, "An Evaluation of the Quantitative and Radiation Features of a Scanning X-ray Transverse Axial Tomograph: the EMI Scanner", Radiology, 111:709-715, June 1974.